.\"  Copyright (C) 2003  Davide Libenzi
.\"  Davide Libenzi <davidel@xmailserver.org>
.\"
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.\" 2007-04-30: mtk, Added description of epoll_pwait()
.\"
.TH EPOLL_WAIT 2 2016-10-08 "Linux" "Linux Programmer's Manual"
.SH NAME
epoll_wait, epoll_pwait \- wait for an I/O event on an epoll file descriptor
.SH SYNOPSIS
.nf
.B #include <sys/epoll.h>
.sp
.BI "int epoll_wait(int " epfd ", struct epoll_event *" events ,
.BI "               int " maxevents ", int " timeout );
.BI "int epoll_pwait(int " epfd ", struct epoll_event *" events ,
.BI "               int " maxevents ", int " timeout ,
.BI "               const sigset_t *" sigmask );
.fi
.SH DESCRIPTION
The
.BR epoll_wait ()
system call waits for events on the
.BR epoll (7)
instance referred to by the file descriptor
.IR epfd .
The memory area pointed to by
.I events
will contain the events that will be available for the caller.
Up to
.I maxevents
are returned by
.BR epoll_wait ().
The
.I maxevents
argument must be greater than zero.

The
.I timeout
argument specifies the number of milliseconds that
.BR epoll_wait ()
will block.
Time is measured against the
.B CLOCK_MONOTONIC
clock.
The call will block until either:
.IP * 3
a file descriptor delivers an event;
.IP *
the call is interrupted by a signal handler; or
.IP *
the timeout expires.
.PP
Note that the
.I timeout
interval will be rounded up to the system clock granularity,
and kernel scheduling delays mean that the blocking interval
may overrun by a small amount.
Specifying a
.I timeout
of \-1 causes
.BR epoll_wait ()
to block indefinitely, while specifying a
.I timeout
equal to zero cause
.BR epoll_wait ()
to return immediately, even if no events are available.

The
.I struct epoll_event
is defined as:
.sp
.in +4n
.nf
typedef union epoll_data {
    void    *ptr;
    int      fd;
    uint32_t u32;
    uint64_t u64;
} epoll_data_t;

struct epoll_event {
    uint32_t     events;    /* Epoll events */
    epoll_data_t data;      /* User data variable */
};
.fi
.in

The
.I data
of each returned structure will contain the same data the user set with an
.BR epoll_ctl (2)
.RB ( EPOLL_CTL_ADD ", " EPOLL_CTL_MOD )
while the
.I events
member will contain the returned event bit field.
.SS epoll_pwait()
The relationship between
.BR epoll_wait ()
and
.BR epoll_pwait ()
is analogous to the relationship between
.BR select (2)
and
.BR pselect (2):
like
.BR pselect (2),
.BR epoll_pwait ()
allows an application to safely wait until either a file descriptor
becomes ready or until a signal is caught.

The following
.BR epoll_pwait ()
call:
.nf

    ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);

.fi
is equivalent to
.I atomically
executing the following calls:
.nf

    sigset_t origmask;

    pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
    ready = epoll_wait(epfd, &events, maxevents, timeout);
    pthread_sigmask(SIG_SETMASK, &origmask, NULL);
.fi
.PP
The
.I sigmask
argument may be specified as NULL, in which case
.BR epoll_pwait ()
is equivalent to
.BR epoll_wait ().
.SH RETURN VALUE
When successful,
.BR epoll_wait ()
returns the number of file descriptors ready for the requested I/O, or zero
if no file descriptor became ready during the requested
.I timeout
milliseconds.
When an error occurs,
.BR epoll_wait ()
returns \-1 and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EBADF
.I epfd
is not a valid file descriptor.
.TP
.B EFAULT
The memory area pointed to by
.I events
is not accessible with write permissions.
.TP
.B EINTR
The call was interrupted by a signal handler before either (1) any of the
requested events occurred or (2) the
.I timeout
expired; see
.BR signal (7).
.TP
.B EINVAL
.I epfd
is not an
.B epoll
file descriptor, or
.I maxevents
is less than or equal to zero.
.SH VERSIONS
.BR epoll_wait ()
was added to the kernel in version 2.6.
.\" To be precise: kernel 2.5.44.
.\" The interface should be finalized by Linux kernel 2.5.66.
Library support is provided in glibc starting with version 2.3.2.

.BR epoll_pwait ()
was added to Linux in kernel 2.6.19.
Library support is provided in glibc starting with version 2.6.
.SH CONFORMING TO
.BR epoll_wait ()
is Linux-specific.
.SH NOTES
While one thread is blocked in a call to
.BR epoll_pwait (),
it is possible for another thread to add a file descriptor to the waited-upon
.B epoll
instance.
If the new file descriptor becomes ready,
it will cause the
.BR epoll_wait ()
call to unblock.

For a discussion of what may happen if a file descriptor in an
.B epoll
instance being monitored by
.BR epoll_wait ()
is closed in another thread, see
.BR select (2).
.SH BUGS
In kernels before 2.6.37, a
.I timeout
value larger than approximately
.I LONG_MAX / HZ
milliseconds is treated as \-1 (i.e., infinity).
Thus, for example, on a system where the
.I sizeof(long)
is 4 and the kernel
.I HZ
value is 1000,
this means that timeouts greater than 35.79 minutes are treated as infinity.
.SS C library/kernel differences
The raw
.BR epoll_pwait ()
system call has a sixth argument,
.IR "size_t sigsetsize" ,
which specifies the size in bytes of the
.IR sigmask
argument.
The glibc
.BR epoll_pwait ()
wrapper function specifies this argument as a fixed value
(equal to
.IR sizeof(sigset_t) ).
.SH SEE ALSO
.BR epoll_create (2),
.BR epoll_ctl (2),
.BR epoll (7)
